1. Field of the Invention
The present invention relates to: an optical pickup device for performing recording/reproduction for a plurality of types of optical disks, such as digital versatile discs (hereinafter referred to as “DVDs”) and Blu-ray discs (hereinafter referred to as “BDs”); and an information processing apparatus incorporating the optical pickup device.
2. Description of the Related Art
A DVD is known as an optical disk that allows for recording with a recording density which is about six times the recording density of a CD, such that large-capacity digital data (e.g., movies or music) can be written thereto. In the recent years, the increasing amounts of information to be recorded have called for optical disks with greater capacities.
In order to increase the capacity of an optical disk, it is necessary to increase the recording density of information. This is generally realized by reducing the spot diameter of laser light which is radiated onto the optical disk during a data write or read. In order to reduce the spot diameter of light, the wavelength of the laser light may be reduced and the numerical aperture (NA) of an objective lens may be increased. In the case of a CD, infrared laser light having a wavelength of 780 nm and a converging element (so-called objective lens) having an NA of 0.45 are used. In the case of a DVD, red laser light having a wavelength of 660 nm and a converging element having an NA of 0.6 are used.
There has also been developed the BD, which, by using blue laser light having a wavelength of 405 nm and an objective lens having an NA of 0.85, allows information to be recorded with a recording density which is five times that of the DVD.
There are several possible structures to be adopted in an information processing apparatus (hereinafter also referred to as an “optical disk apparatus”) which realizes recording/reproduction for both types of optical disks, i.e., BDs and DVDs. One structure involves use of two optical pickup devices, i.e., one dedicated to BDs and another dedicated to DVDs. In this case, the use of separate optical pickup devices results in an increased size of the optical disk apparatus, although designing of each individual optical pickup device may be easy. Another structure employs a single optical pickup device to support all types of optical disks, whereby the optical disk apparatus can be kept the same size as conventionally. As the means for converging laser light in this case, a single objective lens actuator may be used to drive two lenses, i.e., an objective lens for BDs and an objective lens for DVDs, which are switched in accordance with the type of optical disk.
The distance from a point on the surface of an objective lens that lies closest to an optical disk for which recording/reproduction is to be performed (hereinafter referred to as the “proximal point”) to the optical disk surface is known as a working distance (WD). Generally speaking, an objective lens for BDs has a much shorter working distance than that of an objective lens for DVDs. For example, consider a structure in which the proximal point of an objective lens for BDs and the proximal point of an objective lens for DVDs are placed at the same height (along the focus direction) from the surface of an optical disk to be subjected to recording/reproduction, in a state where the actuator is not powered (hereinafter referred to as an “initial position”). With this structure, focus servo can be achieved when performing recording/reproduction for a DVD. However, when performing recording/reproduction for a BD, it is necessary to first bring the objective lens for BDs closer to the optical disk (e.g. by applying a DC current to the actuator), thus moving the objective lens for BDs to a position equal to the WD for BDs, before trying to establish focus servo. FIG. 5 shows a lens holder 302 of this structure. In this lens holder 302, a objective lens 301 for DVDs having a working distance of W1 and an objective lens 300 for BDs having a working distance of WD2 are disposed side by side on the same plane. As shown in (a) of FIG. 5, the lens holder 302 is at its initial position during recording/reproduction for a DVD 200a. However, as shown in (b) of FIG. 5, the lens holder 302 is brought closer to a BD 200b during recording/reproduction for the BD 200b. Specifically, during recording/reproduction for the BD 200b, the lens holder 302 is moved closer toward the optical disk (BD 200b), by a difference (W1−W2) between the respective working distances, from its initial position. Thus, in order to perform recording/reproduction with the lens holder 302 being shifted from the initial position, it is necessary to keep applying a DC current to the actuator within the optical pickup device. This not only requires the actuator to be capable of moving over an enlarged range, but also increases the power consumption of the actuator, corresponding to the offset.
Such an offset can be avoided by disposing the two objective lenses 300 and 301 on the lens holder 302 in such a manner that, by a distance x which is equal to the difference (W1−W2) between the respective working distances, the objective lens 300 for BDs is located closer to the optical disk than is the objective lens 301 for DVDs. FIG. 6 shows a lens holder 402 of this structure. With this structure, in an initial position, the two objective lenses 300 and 301 are at their respective working distances W1 and W2 from the respective optical disks (DVD 200a, BD 200b). Therefore, during recording/reproduction for the BD 200b, it is unnecessary to keep the actuator being offset in a direction toward the BD 200b, whereby increase in the power consumption of the actuator can be avoided.
FIG. 7 is a diagram which more precisely shows relative positions of the objective lenses 300 and 301. In FIG. 7, the difference between the proximal point of the objective lens 300 with respect to the optical disk 200 and the proximal point of the objective lens 301 with respect to the optical disk 200 is defined by a distance “x”.
In the lens holder 402 of FIG. 6, x=W1−W2 is true, so that increase in power consumption due to offset can be avoided. However, as shown in (a) of FIG. 6, the objective lens 300 for BDs is brought closer to the DVD 200a during recording/reproduction for the DVD 200a, thus resulting in an increased likelihood that the objective lens 300 may collide with the DVD when focus servo fails, for example. Moreover, during actual recording/reproduction for the DVD 200a, there may exist “surface runout”, i.e., periodic wobbling (with rotations of the DVD 200a) of the surface of the DVD 200a facing the lens. Therefore, if focus servo should fail during rotation of the DVD 200a, there is a very high likelihood that the DVD 200a may collide with the objective lens 300, given that the amount of surface runout is large. The objective lens 300 for BDs has a short working distance W2. If the value of this short working distance W2 is smaller than the maximum value of the amount of surface runout of the optical disk 200, the aforementioned colliding problem will be prominent.
In order to solve the above problem, Japanese Laid-Open Patent Publication No. 2004-103189 discloses determining the distance x by taking into consideration the maximum amount of surface runout of an optical disk. FIG. 8 shows a lens holder 502 in which the distance x is prescribed so that x<(W1−maximum amount of surface runout). By using the lens holder 502 of this structure, it is possible to prevent collision between the objective lens 300 and the DVD 200a upon failure of focus servo, because the distance between the objective lens 300 and the DVD 200a is greater than the maximum amount of surface runout of the DVD 200a. 
Similarly, Japanese Laid-Open Patent Publication No. 2003-281758 discloses a technique which, in a pickup device having two objective lenses, determines the relative positions of the objective lenses along the focus direction so as to avoid collision between an optical disk and the objective lens having a shorter working distance.
However, the inventors have conducted a detailed study of collisions between the objective lens 100 for BDs and the optical disk 200 by actually operating an optical disk apparatus, which indicated that there are cases where collision cannot be prevented by merely determining the distance x by taking into consideration the surface runout of the optical disk.
In order to prevent collision between the objective lens and the optical disk, it might be possible to provide a protective member around the objective lens, such that the protective member is more protruding toward the optical disk than is a proximal point of the objective lens facing toward the optical disk. However, when performing recording/reproduction at an inner peripheral portion of the optical disk by using the objective lens having a longer working distance, the objective lens having a shorter working distance and the protective member may come in contact with a protrusion (stack rib) which is formed at the inner peripheral portion of the optical disk, thus causing malfunctioning such as servo failure.